Les wrote:
"Why then the propensity towards hull shapes similar to surface vessels?"

Because prior to the Type XXI, submarines were not actually submarines, they were surface vessels with the ability to submerge for brief periods (i.e. they were submersibles, not submarines). Since they spent the vast majority of their time on the surface, it was only logical to design a hull shape which was more efficient on the surface. Prior to 1941, submarines usually dove only to evade a counterattack. They would sometimes dive to make a submerged attack, but it was more common to attack on the surface. For the rare occassions when they submerged, they could put up with a noisier and less hydrodynamic hull shape.
Again, since they were designed as surface warships before WWII, the submarines were given a deck gun (or two) plus some anti-aircraft guns. To accommodate the guns, flat and wide decking and/or platforms around the gun mounts, railings, ready ammo lockers, etc. had to be added. A boat which spends most of its time on the surface needs other external features such as a wide open bridge and the flashing around it to reduce the effects of waves crashing against the conning tower. You also end up with net cutters, jumping wire supports, bollards, cleats, topside torpedo canisters on U-boats, deck planking, etc.
When you design a true submarine which will spend most of its life under water, you design a hull shape which is hydrodynamically effficient underwater even if it is less efficient on the surface since you won't spend much time there. Since you will not be fighting aircraft, you don't need anti-aircraft guns with all of their platforms and gun shields. Since the crew will not be walking around outside of the pressure hull, there is no need for a flat deck or hand rails.
By designing a boat with great underwater endurance, you change the fundamental purpose of the boat from a diving torpedo boat to a submarine. Simply adding a schnorchel to an older design of U-boat (Type VII or Type IX) to extend its underwater endurance helps, but it still leaves the vessel dependant on the surface for a significant portion of its day if it intends to cover any decent distances.